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Improved interfacial properties of carbon fiber/epoxy composites

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Materials Letters 64 (2010) 2742–2744
Contents lists available at ScienceDirect
Materials Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Improved interfacial properties of carbon fiber/epoxy composites through grafting
polyhedral oligomeric silsesquioxane on carbon fiber surface
Feng Zhao, Yudong Huang ⁎
Department of Polymer Science and Engineering, Harbin Institute of Technology, PO Box 410#, Harbin 150001, China
a r t i c l e
i n f o
Article history:
Received 16 July 2010
Accepted 29 August 2010
Available online 15 September 2010
Keywords:
Carbon fiber
POSS
Surfaces
Composite materials
a b s t r a c t
Carbon fibers were grafted with a layer of uniform octaglycidyldimethylsilyl POSS in an attempt to improve
the interfacial properties between carbon fibers and epoxy matrix. Atomic force microscopy (AFM), X-ray
photoelectron spectroscopy (XPS) and dynamic contact angle analysis were performed to characterize the
carbon fibers. AFM results show that the grafting of POSS significantly increased the carbon fiber surface
roughness. XPS indicates that oxygen-containing functional groups obviously increased after modification.
Dynamic contact angle analysis shows that the surface energy of modified carbon fibers is much higher than
that of the untreated ones. Results of the mechanical property tests show that interlaminar shear strength
(ILSS) increased from 68.8 to 90.5 MPa and impact toughness simultaneously increased from 2.62 to 3.59 J.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Carbon fibers are widely used as reinforcements of advanced
composites because of their excellent properties, such as high specific
strength and modulus, light weight and relative flexibility [1].
However, when applied without previous surface treatment, the
physicochemical interaction between carbon fibers and matrix is not
strong enough due to their inert and smooth surfaces, which will
directly affect the interfacial adhesion of the composites [2]. As a
result, extensive research has been devoted to the surface treatment
of carbon fibers in order to increase the quantity of surface functional
groups and enhance the interactions between fibers and matrix, such
as oxidation method [3], electrochemical method [4], plasma
treatment [5], and high energy irradiation [6].
Polyhedral oligomeric silsesquioxanes (POSS) are emerging as a
new chemical technology for the nano-reinforced organic-inorganic
hybrids, which have been applied in diverse areas including
aerospace, semiconductor and biological systems, and becoming the
focus of many studies due to the simplicity in processing and the
excellent comprehensive properties [7]. Although grafting POSS on
the carbon fiber surface is a potential modification method, we could
not find any reports on this in a literature survey.
Here, we grafted octaglycidyldimethylsilyl POSS on the carbon
fiber surface through a series of chemical reactions to enhance the
interfacial adhesion between carbon fibers and matrix. The surface
chemical composition and morphologies of carbon fibers were
investigated by XPS and AFM. Wettability and surface energy of the
⁎ Corresponding author. Tel./fax: + 86 451 86413711.
E-mail address: ydhuang.hit1@yahoo.com.cn (Y. Huang).
0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2010.08.074
carbon fibers were obtained by dynamic contact angle analysis. The
mechanical properties of composites were evaluated by ILSS and
impact toughness.
2. Experimental
PAN-based carbon fibers were purchased from Sinosteel Jilin
Carbon Co., China. Octaglycidyldimethylsilyl POSS was purchased
from Hybrid Plastics. Thionyl chloride (SOCl2), ethylenediamine
(EDA), dimethylformamide (DMF) and tetrahydrofuran (THF) were
purchased from Sigma-Aldrich. The carbon fibers were oxidized in a
3:1 (v/v) mixture of concentrated H2SO4/HNO3 at 60 °C for 2 h. Then
the carboxyl functionalized carbon fibers were reacted with the
mixture solution of 50 ml SOCl2 and 5 ml DMF at 76 °C for 24 h to
yield acyl chloride functionalized carbon fibers. Then the fibers were
reacted with 50 ml EDA at 80 °C for 24 h. After being washed with
deionized water and dried, the amine functionalized carbon fibers
were mixed with 0.5 g POSS in 100 ml THF reacting at 50 °C for 6 h to
obtain POSS grafted carbon fibers. WSR618 epoxy resin and methyl
tetrahyelrophthalic anhydride hardener were supplied by Sinopharm
Chemical Reagent Co., used at a mixture ratio of 100:70. The
unidirectional prepreg of carbon fibers was put into a mold to
manufacture composites. The curing process was at 90 °C for 2 h
under 5 MPa, 120 °C for 2 h under 10 MPa and 150 °C for 4 h under
10 MPa. The resin content of the composites was controlled at 35 ±
1.5 mass%, and the width and thickness of specimens were 6.5 and
2 mm. The surface composition analysis was performed on a Scienta
ESCA 300 XPS. AFM images were obtained using a NT-MDT Solver
P47H system. Dynamic contact angle analysis was performed on a
DataPhysics DCAT21 dynamic contact angle meter and tensiometer.
Mechanical properties were tested on Instron 5569 universal testing
F. Zhao, Y. Huang / Materials Letters 64 (2010) 2742–2744
2743
Table 1
Surface element analysis of carbon fibers.
Carbon fiber
As-received
POSS grafted
Element content (%)
C
O
N
Cl
Si
84.88
66.46
13.50
23.23
1.62
1.90
–
0.10
–
8.31
O/C
Si/C
0.1590
0.3495
–
0.1250
Fig. 1. Structure of octaglycidyldimethylsilyl POSS.
machine and Instron 9250HV drop weight impact test system. The
structure of octaglycidyldimethylsilyl POSS is shown in Fig. 1.
3. Results and discussions
The AFM images of untreated, POSS grafted carbon fibers are
shown in Fig. 2. Remarkable differences of the surface topography can
be observed between the untreated and modified carbon fibers. As
shown in Fig. 2a, the surface of the untreated carbon fiber seems to be
relatively neat and smooth, and a few narrow grooves parallel
distribute along the longitudinal direction of the fiber. After
modification, a layer of POSS particles are grafted uniformly on the
fiber surface and the roughness increases obviously (Fig. 2b). The
increased surface roughness can significantly increase the interfacial
adhesion by enhancing mechanical interlocking between the fiber and
the matrix.
The results of the carbon fiber surface composition obtained by
XPS were summarized in Table 1. It is found that the elements of the
untreated carbon fiber surface only include carbon, oxygen and
insignificant amount of nitrogen. After being grafted with POSS, the
carbon content decreased from 84.88% to 66.46% and the oxygen
content increased significantly from 13.50% to 23.23%. In addition,
significant silicon elements of 8.31% were detected on the fiber
surfaces, and surface atomic O/C and Si/C ratios increased sharply,
which were due to the Si–O cage structure and the epoxy groups of
POSS. These numerous epoxy groups can effectively increase resin
compatibility and react with matrix.
The changes of chemical environment and topography of carbon
fiber surfaces affect the fiber surface energy as well as its components.
In Table 2, the advancing contact angle (θ), the surface energy (γ), its
dispersion component (γd) and polar component (γp) of the
untreated and POSS grafted carbon fibers are summarized. As
shown in Table 2, the surface energy of untreated fibers was
43 mN m− 1, with a dispersion component of 36 mN m− 1 and a
polar component of 7 mN m− 1. After modification, obvious decreasing trends of contact angles were observed from the untreated fibers
to the POSS modified fibers for both the polar water and the non-polar
diiodomethane. The contact angles decreased from 73.21 to 53.68° for
water and from 46.85 to 42.52° for diiodomethane. In addition, the
surface energy and its components of POSS grafted carbon fibers
obviously increased compared with those of the untreated fibers. The
increased polar component of modified fibers was due to the epoxy
groups of POSS on the fiber surface, and the increased dispersion
component was due to the increased roughness caused by POSS
particles and the different surface composition of carbon fibers. The
increased surface energy can effectively improve the wettability of the
fibers by the resin and increase the interfacial strength.
The mechanical property testing results of the composites
reinforced by different carbon fibers are shown in Fig. 3. From
Fig. 3a, it can be clearly seen that the grafting of POSS significantly
increased the interfacial strength of the composites. The ILSS
increased from 68.8 to 90.5 MPa by 31.5%. The improvement of the
interfacial strength could be attributed to the enhancement of the
mechanical interlocking and chemical bonding between the fibers and
matrix. After modification, the epoxy groups of POSS play an
important role in improving the interfacial adhesion between the
fibers and matrix. In addition, the rigid POSS particles grafted on the
fiber surfaces can greatly enhance the mechanical interlocking with
the resin.
The impact property testing results are shown in Fig. 3b. The initial,
propagative and total absorbed energy of untreated carbon fiber
composites were 0.45, 2.17 and 2.62 J, respectively. After modification,
the impact properties of POSS grafted carbon fiber composites
increased. The initial, propagative and total absorbed energy increased
to 0.71, 2.89 and 3.59 J, by 57.8%, 33.2% and 37.0%, respectively. When
the composites are under load, POSS in the composite interface can
induce more cracks which can efficiently absorb the fracture energy,
resulting in the increase of the initial absorbed energy. After crack
formation, POSS can efficiently change the direction of the crack
propagation, which increases the propagative absorbed energy. In a
word, the stress concentration around POSS, the inducement of cracks
Table 2
Contact angles and surface energy of carbon fibers.
Fig. 2. AFM topography images of carbon fibers. a) untreated, and b) POSS grafted.
Carbon
fiber
θwater
(°)
θdiiodomethane
(°)
γd
(mN m− 1)
γp
(mN m− 1)
γ
(mN m− 1)
As-received
POSS grafted
73.21
53.68
46.85
42.52
36.00
38.32
7.00
16.55
43.00
54.87
2744
F. Zhao, Y. Huang / Materials Letters 64 (2010) 2742–2744
between carbon fibers and epoxy matrix. The roughness of carbon
fiber surface significantly increased after being grafted with octaglycidyldimethylsilyl POSS. These POSS particles on the fiber surface
provided a means to enhance the mechanical interlocking with the
resin. XPS results indicate that the oxygen-containing functional
groups obviously increased after modification, which is beneficial to
increase the chemical bonding between carbon fibers and matrix. The
surface energy of the modified fibers is much higher than that of the
untreated ones, which can lead to better wettability by the resin and
better interfacial adhesion. The mechanical performance tests show
that the introduction of octaglycidyldimethylsilyl POSS in the
interface region can obviously improve the interfacial strength and
simultaneously increase the impact toughness. This significant
improvement of mechanical properties is obtained without any effort
to optimize the grafting reaction conditions, so it is expected that the
interfacial properties can be further increased.
Acknowledgement
The authors gratefully acknowledge financial supports from the
Chang Jiang Scholars Program, the National Natural Science Foundation of China (No. 51073047) and the National Natural Science
Foundation of China (No. 51003021).
References
Fig. 3. Mechanical properties of the composites reinforced by untreated and POSS
grafted carbon fibers. a) ILSS and b) impact toughness.
and the crack propagation orientation deflection are helpful to
improve the impact properties of the composites.
4. Conclusion
Carbon fibers were grafted with a layer of uniform octaglycidyldimethylsilyl POSS in an attempt to improve the interfacial properties
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